Pivot joint

ABSTRACT

A pivot joint has a first structure comprising a ball mounted in a receptacle, wherein the location of the ball is held with respect to the receptacle, and a second structure mounted on and movable with respect to the ball, wherein the second structure has at least one bearing surface which defines its position with respect to the ball. The ball may be held in location by contact with the receptacle at least two positions. At least one of the positions of contact may be adjustable.

This is a Continuation of application Ser. No. 11/798,462 filed May 14,2007, which is a Continuation of application Ser. No. 10/752,680 filedJan. 8, 2004, that issued as U.S. Pat. No. 7,241,070, which in turn is aContinuation-in-Part of International Application No. PCT/GB02/03294filed Jul. 15, 2002. The disclosure of the prior applications is herebyincorporated by reference herein in its entirety.

The present invention relates to a pivot joint, in particular a highprecision ball joint.

Pivot joints which comprise a ball on a stalk are known. In such pivotjoints the ball is located within a socket with the stalk protrudingfrom the socket. These pivot joints have the disadvantage that they arenot high precision, particularly because the ball part is not accuratelyspherical. A further disadvantage is that parts cannot be cheaplyreplaced when worn. As such joints wear, their positional accuracydecreases which can result in undesirable movement at the pivot point.

Accurate spheres (i.e. ball bearings) can be made very precisely by alapping process.

In a first aspect the present invention provides a pivot jointcomprising:

a first structure comprising a ball mounted in a receptacle, wherein thelocation of the ball is held with respect to the receptacle; and

a second structure mounted on and movable with respect to the ball,wherein the second structure has at least one bearing surface whichdefines its position with respect to the ball.

The ball is preferably held in location by contact with the receptacleat least two positions.

The at least two positions of contact are preferably on opposite sidesof the ball. They may be diametrically opposite, but this is notessential and in some circumstances, for example where a precision jointis required it is preferred that the positions of contact are formed asthe apexes of triangles whose centre passes through the ball.

Preferably, the second structure is captured between the receptacle andthe ball. Thus, the receptacle and second structure are interconnectedvia the ball. In this configuration, at least one contact is on eitherside of the second structure i.e. at least one contact is on each sideof the bearing surface which defines the position of the secondstructure with respect to the ball.

In a preferred embodiment, at least one of the positions of contact isadjustable.

Preferably the ball is held or fixed in location by at least four pointsof contact with the first structure.

When the ball is held in location, it may be movable with respect to thereceptacle as well as the second.

However, when a high precision joint is required, the ball needs to bestationary with respect to the first structure and thus is fixed inposition.

By having at least one adjustable position of contact, either of thesestates may be used, as required.

Alternatively, the ball is held in location with respect to the firststructure or fixed by a conical recess and at least one point ofcontact.

Preferably the second structure has three bearing surfaces.Alternatively the second structure may have two bearing surfaces.

In a preferred embodiment, the second structure is biased into contactwith the ball. Such a bias may be provided by a spring, magnetic force,a pressure differential (e.g. vacuum) or elastic band, for example.

A second aspect of the invention provides a method of holding a locationof a ball with respect to a structure wherein the ball is sited in saidlocation by contacting the structure at least two positions.

Preferably, the at least two positions are on opposite sides of theball.

In preferred embodiments the ball is fixed or held in location by atleast four points of contact or, alternatively, by a conical recess andat least one point of contact.

A third aspect of the invention provides a method of locating astructure on a ball wherein the structure is movable with respect to theball, the structure comprising an aperture with a peripheral surface forreceiving the ball wherein at least one area of the peripheral surfaceis configured to be in sliding contact with the ball.

Preferably, at least two areas of the peripheral surface are configuredto be in sliding contact with the ball and areas of the peripheralsurface between said at least two areas are configured not to be incontact with the ball.

A fourth aspect of the invention provides a pivot joint comprising:

a first structure comprising a ball retained in a receptacle; and

a second structure movable with respect to the first structure and incontact with the ball via a bearing, the contact defining location of apivot point characterised in that:

the bearing is shaped to receive the ball wherein, as the bearing wears,the pivot point is substantially maintained in its defined location.

An advantage of a pivot joint according to the invention is thatundesirable movement in the plane of the pivot joint is mitigated thusthe joint has minimal longitudinal movement and the overall length ofthe structures connected via the pivot joint remains substantially thesame over time. This means that the joint is less susceptible to theeffects of wear and may remain within tolerance levels for longerincreasing either the lifetime of the joint or the time between thereplacement of parts of the joint.

Preferably, a bias is used to help to ensure that the second structureis at least partially received on the bearing at the correct position.This will result in the wearing process on the bearing occurring at thedesired places in order to maintain the pivot point at substantially itsdefined location.

The ball is preferably a precision ball for example a lapped ball suchas a ball bearing. The ball, even when retained in the receptacle, isindependent i.e. not fixed in location by use of a screw, stud or othermeans which affects sphericity of the ball and thus accuracy. Such anindependent ball may act as a precision bearing and in addition,provides the location point of the joint. The ball is retained in thereceptacle by clamping, gluing, welding or braising for example, othermethods which do not distort the shape of the ball are also appropriate.

Ball bearings with highly spherical surfaces can be used to produce asmooth and precise motion between two structures that have not undergoneexpensive accurate machining processes. One of the reasons that thesepivot joints are so accurate is because the ball may be held or fixed inlocation without the use of a screw stud which would affect thesphericity and smoothness of the ball surface and joint movement.

In a preferred embodiment, there is provided a third structure moveablewith respect to the first and second structures whereby the thirdstructure is in contact with the ball via a further bearing, the contactdefining the location of a pivot point which is substantially maintainedin its defined location as the bearing surface wears.

The bearing may comprise an air bearing, a roller bearing or a planebearing. Thus, the bearing may be provided by a surface of the first(and third) structure. Alternatively, a separate bearing, for examplecomprising a number of ball bearings in a race, provides the contactbetween the structures.

Pivot joints according to the invention enable the production of (high)precision joints at low cost.

Embodiments of the invention will now be described by way of example andwith reference to the accompanying drawings in which:

FIG. 1 is an isometric view of a coordinate measuring machine using thepivot joint of the invention;

FIG. 2 is an isometric view of the upper stage of the coordinatemeasuring machine, showing the pivot joints;

FIG. 3 is a plan view of one end of a strut of the coordinate measuringmachine;

FIG. 4 is a plan view of the upper stage of the coordinate measuringmachine, showing the pivot joint;

FIG. 5 is a section along line A-A of FIG. 4;

FIGS. 6 a and 6 b are views of an end of an alternative strut of thecoordinate measuring machine;

FIGS. 7 and 8 are schematic illustrations of alternative clampingarrangements of a ball in a pivot joint;

FIG. 9 is an isometric view of the upper stage of the coordinatemeasuring machine, showing alternative pivot joints;

FIGS. 10 a, 10 b and 10 c show isometric views of alternate pivot jointsaccording to the invention; and

FIG. 10 d shows a cross-section through plane A-A of the pivot jointshown in FIG. 10 a.

A coordinate measuring machine using pivots joints according to theinvention is shown in FIG. 1. The coordinate measuring machine comprisesa lower fixed stage 10 and an upper movable stage 12. The upper andlower stages are linked by telescopic struts 14, each strut beingconnected to the upper and lower stages at its respective upper andlower ends by pivot joints. Each strut has a motor 16 to increase ordecrease its length.

As the upper stage 12 is supported only by the three telescopic struts14 which are connected to the upper and lower stages by pivot joints,this stage may rotate about three perpendicular axes relative to thelower stage 10. To prevent this, three anti-rotational devices 20 areprovided which eliminate these three degrees of rotational freedomwhilst allowing translational movement. The devices are passive, i.e.they have no motor or other actuator. The joints between theanti-rotational devices 20 and the upper and lower stages 12,10 are alsopivot joints. The upper stage 12 comprises the receptacles and balls(first structures) for a number of pivot joints. Each strut 14 andanti-rotational device 20 forms a second structure (it is not essentialfor all the joints to be according to the invention).

A pivot joint between the upper stage 12 and an anti-rotational device20 is shown in more detail in FIGS. 2, 4 and 5. The upper stage 12 isprovided with structured cut-outs 30. A ball 32 of the pivot joint issupported within the cut-out 30 by surfaces defining the periphery ofthe cut-out 30. Thus, the first structure is provided by the upper stageplus the ball. Two cut-outs 30 are shown in FIG. 2, one with the othercomponents of the ball joint in place and the other without.

The ball 32 is supported by two opposite surfaces 34,36 of the cut-out30. A first curved surface 34 contacts a side of the ball 32, and asecond curved surface 36 contacts a lower surface of the ball 32 withtwo points of contact 36A,36B. The position of the ball 32 is defined bythese three points of contact 34,36A,36B and a fourth point of contactprovided by a clamp 38,42. The ball is thus kinematically and repeatablylocated within the cut-out.

In this example the clamp comprises a washer 38 and a fixing device 42,such as a screw or bolt. The washer 38 sits on a shelf 41 provided inthe cut-out 30 in the upper stage 12, an edge of its lower surface incontact with the ball 32. The upper stage 12 is provided with a hole 40aligned with the centre of the washer 38, to receive the fixing device42 which holds the washer 38 against the ball 32. The ball 32 is thusheld rigidly in a fixed position at four places around its surface bythe first curved surface 34, by two points of contact 36A, B on thesecond curved surface 36, and by the washer 38.

An alternative pivot joint between the upper stage 12 and a strut 14 isshown in FIG. 9. The upper stage 12 is provided with structured cut-outs130. A ball 32 of the pivot joint is supported within the cut-out 130 bysurfaces defining the periphery of the cut-out 130. Two cut-outs 130 areshown in FIG. 9, one with the other components of the ball joint inplace and the other without.

The ball 32 is supported by two opposite surfaces 134,136 of the cut-out130. A first curved surface 134 contacts a lower surface of the ball 32,and a second curved surface 136 contacts a side of the ball 32 with twopoints of contact 136A,136B. The position of the ball 32 is defined bythese three points of contact 134,136A,136B and a fourth point ofcontact provided by a clamp 38,42.

The orientation of the contacting surfaces, for the cut-out 130 arerotated with respect to the cut-out 30 as the orientation of the strut14 is rotated with respect to anti-rotational device 20. This can beseen in FIG. 1.

Each end of each of the telescopic struts 14 and anti-rotational devices20 is provided with a hole 45 which fits over a ball 32 of a respectivepivot joint. Each hole 45 is shaped to have three bearing surfaces 43A,B, C, spaced at 120° apart, as shown in FIG. 3. Only the three bearingsurfaces are in contact with the ball 32.

These bearing surfaces have small areas of contact with the ball 32 tomaximise slide between the surfaces.

These small areas of contact also provide repeatable positioning of thestrut 14 or anti-rotational device 20 with respect to the ball 32.

For FIG. 3, the three bearing surfaces are planar through the thicknessof the structure and curved through the plane of the structure so as todescribe part of a cone which has a surface defined by the bearingsurfaces i.e. they are conical in shape. The ball sits on these threesmall areas of contact.

A second structure 20 is provided with a hole 45 which partly receivesand fits over a ball 32 of a respective joint.

The ball is retained in a receptacle and acts as a precision surface forthe centre of the bearing and, in addition, the ball provides the linkbetween the two parts of the joint.

Although the examples show the second structure 14,20 encircling theball 32, this is not essential. Thus, the second structure maysubstantially encircle the ball with a gap in the structure (thelimiting size of any gap is a function of the diameter of the ball andthe position of the bearing surfaces of the second structure).

It should be noted that whilst it is preferred that both the struts 14and anti-rotational devices 20 have pivot joints according to theinvention, this is not essential.

FIG. 6 a shows an alternative arrangement of the bearing surfaces. Inthis arrangement the hole 45 is oval with only two bearing surfaces46A,46B. As the bearing surfaces wear, they sit further down onto thesurface of the ball, thus reducing rattle between the bearing surfacesand the ball. This arrangement is particularly suitable when the forceon the bearing surface is parallel to an axis perpendicular to the planeof the bearing surfaces (for example if the ball joint is used in ajack).

For FIG. 6, the two bearing surfaces are curved in two dimensions.Firstly through the thickness of the second structure to partiallyreceive the ball (FIG. 6 b) and, secondly in the longitudinal directionof the structure (FIG. 6 a) where the width of the bearing surfaces isat a maximum at the position where it is desired to seat the ball (inthis case this is central to the oval aperture) tapering off either sideof this desired position. This tapering encourages the ball to seat inthe desired position.

The struts 14 and anti-rotational devices 20 are formed from a goodbearing material, such as phosphor bronze. The strut 14 may be made byetching and then machining or stamping the holes 45 to accuratelycontrol the length and geometry of the holes 45.

A side view of the upper stage 12 with an anti-rotational device 20mounted on a pivot joint is shown in FIG. 5 which is a section alongline A-A of FIG. 4. The ball 32 can be seen to be held in position bythree of the four points of contact. These are the first curved surfaces34 and two points of contact on the second curved surface 36 provided onthe stage 12 and the surface 37 of the washer 38. These surfaces areshown to be spaced around the circumference of the ball 32. The surface37 of washer 38 in contact with the ball 32 is a chamfered edge betweenthe lower and side surfaces of the washer 38 and extends all around thecircumference of the washer 38 so that this surface 37 is presented tothe ball 32 whatever the orientation of the washer 38.

In this Figure the anti-rotational device 20 is in position and onebearing surface 43 can be seen to be in contact with the ball 32. Theother two bearing surfaces cannot be seen.

The location of the four points of contact between the ball 32 and upperstage 12 and washer 38 allows sufficient space for the end of theanti-rotational device 20 to move about the ball 32. In addition, thestructured cut-outs 30 each have cut-out lobes 31 either side of theball 32 to allow enough room for the end of the anti-rotational device20 to manoeuvre. The three bearing surfaces 43A, B, C remain in contactwith the ball yet move over the ball to allow the arm of theanti-rotational device 20 to rotate about the ball 32.

In the event of a jolt the bearing surfaces will spring off the ball,hit a solid stop (i.e. the edges of the cut-out structure 30) and returnto their position on the ball. The ball joint is thus self-protecting.Where high precision ball joints are required, coil springs may beprovided, for example between the two parts of an anti-rotational device20, to urge the bearing surface (s) onto the ball 32. Such springs wouldassist in returning the bearing surfaces onto the ball after any joltand add rigidity to the structure. Such springs would also aid inmaintaining the location of the ball with respect to the bearingsurface. Thus, as the bearing surfaces wear, the ball would remain(quasi-) kinematically located in position maintaining a constant lengthof strut. The person skilled in the art will appreciate that biasingmeans would be advantageous at other locations on an apparatusincorporating a ball joint according to the invention.

In this embodiment, where the ball is retained and supported by thecut-out, the ball remains located with respect to the joint when thestrut (or second structure) is not engaged with the ball at the bearing.

The use of a bias ensures that the ball repeatedly contacts the bearingsurfaces at the correct location, according to the shape of the bearingsurface, to minimise lateral movement of the ball with respect to thesecond structure as the bearing surface wears. Thus, the active lengthof a strut 14 or anti-rotational device 20 remains stable over time andwith wear of the machine. The active length is the length between thecentre of the pivot joints at each end of the structure 14,20. If thisactive length were not maintained, then after a period of use, themachine would not position itself where dictated by the machine motorsas there would be lateral movement in the structure which would not bethe same for each joint. For struts 14, this would affect the positionof the upper stage 12 in the x, y or z direction. For anti-rotationaldevices 20, this affects the rotation of the upper stage 12 with respectto the lower fixed stage 10 and the degree of parallelism achievedbetween the two stages. This pivot joint has the advantages that it isboth cheap to manufacture and easy to assemble.

The cut-out structure which holds the ball is easy to machine.Repeatable (kinematic) positioning of a free ball within this structureenables the ball to be easily replaced. This also enables correctgeometric positioning of the parts on assembly of the machine.

Furthermore use of commercially available accurate spheres (for exampleball bearings) combined with precise positioning in the structureproduces a high precision ball joint which ensures a good seatingposition of the ball within the aperture of the second structure. Theball may be retained in position by clamping, gluing, welding or anyother method which maintains the integrity of the ball as a sphere.

An alternative way of urging the bearing surfaces onto the ball 32 is bybiasing the second structure 14,20. One way to achieve this is toprovide a plate which has one end clamped to the second structure 14,20and the other end pressing against the ball 32. The plate thus acts likea leaf spring. The clamp is preferably removably fixed by, for example,a screw, to enable removal of the ball 32 for maintenance or replacementthereof. The word plate is intended to include all the various shapesand structures that are suitable for providing a bias such as a washer.Such a plate could be used in conjunction with washer 38 or as areplacement.

When a bias is used, it ensures that all the contact points between theball and the second (and also third) structures are equally rigid andloaded so all contacts have equal wear rates.

The ball used in the pivot joint may be made of any hard/low frictionmaterial or alternatively made of any material coated with a hard/lowfriction material.

Although the above example describes the use of a pivot joint in acoordinate measuring machine, this pivot joint is not limited to usewith such machines and may be used in other applications. Examples ofother applications include positioning machines such as robots, machinetools, measuring machines, also non-cartesian mechanisms and otherparallel kinematic machines.

Furthermore the invention is not limited to the method of locating aball as described above. Although the embodiment describes four pointsof contact with the ball, one of which is adjustable, it is possible tohave more than a total of four points of contact with the ball. Inaddition it is possible to have more than one adjustable point ofcontact, for example all four points of contact could be adjustable.

An alternative method of clamping a ball rigidly in a precise locationis shown in FIG. 7. The ball 32 is located in a conical recess 48 andheld in place by a securing device 52 (for example a screw) with onepoint of contact 54 with the ball 32. Of course the securing device 52may have more than one point of contact with the ball 32 (for example av-groove).

FIG. 8 shows another method of clamping a ball rigidly in a preciselocation. The ball 32 is located on a flat surface 50 and is held inplace by a securing device 52 which has a conical recess 56 in contactwith the surface of the ball 32.

The bearing surfaces are preferably sloped, at a shallow angle allowingthe ball to sit on the surfaces (see FIG. 6 b). An angle of betweenabout 15° and 45° to a line perpendicular to the plane of the bearingsurfaces is suitable. Thus, each of the surfaces of the hole adapted toreceive the ball describes a cone. The surfaces may describe a singlecone or different, non-concentric cones where the contact radius of eachcone is greater than the radius of the ball. An angle of less than about15° may result in the ball becoming jammed in the hole which, of course,affects the mobility of the joint. An angle of greater than about 45°requires increasingly large loads to force the ball to be received inthe hole. The preferred angle is about 15-30° with the range of about15-20° most preferred as this shallow angle provides a good compromisebetween the sliding action of the bearing surface with respect to theball and the ease with which the ball is received within the hole. Ashallow angle is useful if the bearing surfaces are urged together by aspring as it enables a weaker spring to be used than if steeper anglesare used.

As an alternative to using bearing surfaces of one of the structures, aseparate bearing is housed between the structures. In this example, itis preferred that in addition to the bearing surface which receives theball of the first structure being shaped to achieve this, that both theother bearing surface and the communicating surface of the secondstructure are also so shaped. This shaping assists in maintaining thepivot joint in its location. If the bearing is an air bearing, then thedistance between the two structures should be maintained at asubstantially constant value thus, the surfaces of the first and secondstructures which cooperate are required to have co-operating forms.Thus, the ball may float between the receptacle and second structure andspin freely with respect to both.

Regardless of the type of bearing used, it is preferred that the firstand second structures are manufactured relative to each other.

For a high precision joint, a bias may be used to force the secondstructure into contact with the ball. For lower precision applications,this may be dispensed with by carefully selecting the angle of thebearing surface.

FIGS. 10 a and 10 b show isometric views of pivot joints 100 and 101respectively. The first structure is provided by a receptacle 106 for aball and a ball 114.

Second and third structures are provided by two struts 102,104 which areeach provided with a hole 122,124 respectively near an end which partlyreceive and fit over the ball 114. The two struts 102,104 are locateddiametrically opposite each other on the ball 114. The distal ends ofthe second and third structures are also provided with pivot joints.These distal pivot joints can be of the single strut variety or doublestrut joints where either the same two struts, or one of these strutsand a different strut coincide.

In this example, the holes 122,124 in the struts are each shaped asrounded triangles (see FIG. 10 c) in order to provide three bearingsurfaces 126A, B, C—one on each side of the triangular structure—incontact with the ball. The bearing surfaces are angled so are notperpendicular to the longitudinal axis 128 of the struts. This allowsthe ball 114 to be seated (partially) within the holes 122,124.

In order to ensure that the two struts 102,104 each contact the ball atthe three bearing surfaces, the two struts are biased together. FIG. 10a shows two u-shaped springs 108 which act on opposite sides of thestruts. Each spring 108 connects between a lug 112 on the struts pullingthe struts together and the bearing surfaces onto the ball surface. FIG.10 b shows an alternative arrangement where two coil springs 110 areconnected across the lugs 112 of the two struts (the second coil springnot shown).

Alternatively, the springs are located between facing sides of thestruts either near the holes in the struts or, if a stronger spring isused, about halfway along the length of the struts (this may enable thebiasing of both ends of the struts using one spring)

As opposed to using a spring to bias the bearing surfaces into contactwith the ball surface, magnetic attraction can be used. Here the lug oneach strut houses or includes a magnet. The magnets are positioned suchthat opposite poles face the gap between the two struts thus the twostruts are magnetically attracted and locate correctly around the ball.

As an alternative to the struts being pulled towards each other, thestruts can be each be pushed towards the bearing surface and each otherby a bias (spring, magnetic force, pressure differential etc) on thereceptacle.

The ball 114 is supported in the receptacle 106 by clamping between tworeceiving surfaces 132,134 provided on one side by the receptacle 106 a(see FIG. 10 d) and, at an approximately diametrically opposed point ofthe ball, by a grub screw 130 which is rotatably mounted within thereceptacle 106 b. The use of one movable receiving surface 134 enableseasy positioning of the ball 114 with respect to the two struts 102,104and the receptacle 106 a,106 b. Only the receptacle and ball have beenshown in FIG. 10 d for clarity.

Alternatively, the ball may be push-fitted into a correspondingly sizedgap in the receptacle. Or, the ball may be mounted on a central rod (notshown) which passes through the centre of the ball 114 and into thereceptacle 106 on either side of it, the rod exits the receptacle on oneside (not shown) and is clamped ensuring that the ball 114 is fixed inthe receptacle 106.

A double strut pivot joint such as has been described has a number ofapplications, one of these is in a coordinate measuring machine. Such amachine comprises two platforms which are linked by at least threestruts which are of changeable length. An example is a hexapod which isdescribed more fully in International Patent Application WO95/20747.

The ball may also be held in position by permanent methods such asbonding, braising and welding. However these methods have thedisadvantage that the ball is then no longer replaceable.

This pivot joint has the advantages that it is both cheap to manufactureand easy to assemble.

The cut-out structure which holds the ball is easy to machine.Repeatable positioning of a free ball within this structure enables theball to be easily replaced.

Furthermore use of commercially available accurate spheres (for exampleball bearings) combined with precise positioning in the structureproduces a high precision ball joint.

The use of the word pivot in the phrase pivot joint throughout thisspecification is defined as including pivot joints with one, two andthree degrees of freedom as well as universal joints having two or threedegrees of freedom.

1. A co-ordinate positioning or measuring machine, comprising: a firstplatform; a second platform; and at least a first strut, wherein thefirst platform is connected to the second platform via the first strut,the first platform comprises a first receptacle that retains a firstball, wherein the first ball is held in a fixed location relative to thefirst receptacle, the first strut has a first end defining a firstaperture having a peripheral surface, the peripheral surface of thefirst aperture being mounted on and movable with respect to the firstball.
 2. A machine according to claim 1, wherein the second platformcomprises a second receptacle that retains a second ball, the secondball is held in a fixed location relative to the second receptacle.
 3. Amachine according to claim 2, wherein the first strut has a second enddefining a second aperture having a peripheral surface, the peripheralsurface of the second aperture being mounted on and movable with respectto the second ball.
 4. A machine according to claim 1, wherein the firstplatform comprises a first clamp, the first ball is held in the firstreceptacle by the first clamp.
 5. A machine according to claim 1,wherein the first ball is held stationary relative to the firstreceptacle.
 6. A machine according to claim 1, wherein the peripheralsurface of the first aperture comprises at least two spaced apartbearing surfaces in sliding contact with the first ball.
 7. A machineaccording to claim 6, wherein the peripheral surface of the firstaperture comprises three spaced apart bearing surfaces in slidingcontact with the first ball.
 8. A machine according to claim 6, whereinareas of the peripheral surface of the first aperture that are locatedbetween the at least two spaced apart bearing surfaces do not contactthe first ball.
 9. A machine according to claim 6, wherein each bearingsurface is angled relative to a line perpendicular to the plane thatcontains the bearing surfaces.
 10. A machine according to claim 1,wherein the first ball comprises a ball bearing made by a lappingprocess.
 11. A machine according to claim 1, wherein the first strut isa telescopic strut that comprises a motor for changing its length.
 12. Amachine according to claim 1, wherein the first aperture is biased intoengagement with the first ball.
 13. A machine according to claim 1,wherein a second strut and a third strut are also provided, the firstplatform also being connected to the second platform by the second strutand the third strut.
 14. A co-ordinate positioning or measuring machine,comprising: a first platform; a second platform; and at least a firststrut connecting the first platform and the second platform, wherein thefirst platform is attached to the first strut via a first pivot joint,the first pivot joint comprising a first structure and a secondstructure, the first structure comprising a ball and a receptacle, theball is held in a fixed location relative to the receptacle, the secondstructure defining an aperture having a peripheral surface, theperipheral surface being mounted on and movable with respect to theball.
 15. A machine according to claim 14, wherein the receptaclecomprises a cut-out provided on the first platform.
 16. A machineaccording to claim 14, wherein the first strut provides the secondstructure that defines the aperture.
 17. A co-ordinate positioning ormeasuring machine, comprising: a first stage; a second stage; and aplurality of struts that connect the first stage and the second stage,wherein each of the plurality of struts comprises a first end and asecond end, the first and second ends of each strut are connected to thefirst and second stages respectively by a plurality of pivot joints,each of the plurality of pivot joints comprises a first structure and asecond structure, the first structure comprises a ball and a receptacle,the ball being held in a fixed location relative to the receptacle, andthe second structure defines an aperture having a peripheral surface,the peripheral surface being mounted on and movable with respect to theball.
 18. A machine according to claim 17, comprising three struts thatconnect the first stage and the second stage.
 19. A machine according toclaim 17, wherein each of the plurality of struts comprises a telescopicstrut of changeable length.
 20. A machine according to claim 17, whereinthe first stage is also connected to the second stage by anti-rotationaldevices, the anti-rotational devices eliminating the three degrees ofrotational freedom between the first and second stages.